Crystalline polypropylene resin-based cellular articles are, as being characterized in that (1) their buffer capacity is high, (2) their thermal conductivity is low, and their thermal stability is good as their heat-resisting temperature is much higher than that of polyethylene cellular articles, (3) their chemical resistance is good, (4) their machine workability, press workability and vacuum moldability are good, and (5) they are stiff and have high strength, much used, for example, in buffer materials, wrapping materials, bumper core materials, floating materials, heat-insulating materials, etc.
For producing such crystalline polypropylene resin-based cellular articles, in general, preferably employed is a method comprising dispersing resin beads in an aqueous medium containing a dispersant, then adding a physical foaming agent thereto under heat and pressure to thereby introducing the foaming agent into the resin beads, and thereafter releasing the resin beads along with the aqueous medium into an open space at an atmospheric pressure for primary foaming into foaming beads. In that method, the foaming beads thus prepared are heated in a mold under pressure generally with steam for secondary foaming into a cellular article having a desired shape.
In the foaming method of that type, it is desirable from the viewpoint of industrial advantages to attain the primary foaming and the secondary foaming at temperatures as low as possible for preparing foaming beads having a desired expansion ratio and for finally producing high-quality cellular articles having a high expansion ratio but having few voids.
Crystalline propylene-ethylene random copolymers having a larger ethylene unit content will generally have lower rigidity. Therefore, in order to increase their rigidity, the ethylene unit content of the copolymers must be lowered. However, for conventional crystalline propylene-ethylene random copolymers having a lowered ethylene unit content, the heating temperature for the primary and secondary foaming shall be inevitably high, thereby bringing about some problems with the equipment for the foaming process, and the high heating temperature is disadvantageous for industrial-scale production of cellular articles of the copolymers.